1. Field of the Invention
The present invention relates to tissue-supporting medical devices, and more particularly to expandable, non-removable devices that are implanted within a bodily lumen of a living animal or human to support the organ and maintain patency, and that have openings for delivery of a plurality of beneficial agents to the intervention site as well as a surface coating of an antithrombotic agent.
2. Discussion of the Related Art
In the past, permanent or biodegradable devices have been developed for implantation within a body passageway to maintain patency of the passageway. These devices are typically introduced percutaneously, and transported transluminally until positioned at a desired location. These devices are then expanded either mechanically, such as by the expansion of a mandrel or balloon positioned inside the device, or expand themselves by releasing stored energy upon actuation within the body. Once expanded within the lumen, these devices, called stents, become encapsulated within the body tissue and remain a permanent implant.
Known stent designs include monofilament wire coil stents (U.S. Pat. No. 4,969,458); welded metal cages (U.S. Pat. Nos. 4,733,665 and 4,776,337); and, most prominently, thin-walled metal cylinders with axial slots formed around the circumference (U.S. Pat. Nos. 4,733,665; 4,739,762; and 4,776,337). Known construction materials for use in stents include polymers, organic fabrics and biocompatible metals, such as, stainless steel, gold, silver, tantalum, titanium, and shape memory alloys, such as Nitinol.
U.S. Pat. No. 6,241,762, which is incorporated herein by reference in its entirety, discloses a non-prismatic stent design which remedies several performance deficiencies of previous stents. In addition, preferred embodiments disclosed in this patent provide a stent with large, non-deforming strut and link elements, which may contain holes without compromising the mechanical properties of the strut or link elements, or the device as a whole. Further, these holes may serve as large, protected reservoirs for delivering various beneficial agents to the device implantation site without the need for a surface coating on the stent.
Of the many problems that may be addressed through stent-based local delivery of beneficial agents, one of the most important is restenosis. Restenosis is a major complication that may arise following vascular interventions such as angioplasty and the implantation of stents. Simply defined, restenosis is a wound healing process that reduces the vessel lumen diameter by extracellular matrix deposition and vascular smooth muscle cell proliferation and which may ultimately result in renarrowing or even reocclusion of the lumen. Despite the introduction of improved surgical techniques, devices and pharmaceutical agents, the overall restenosis rate for bare metal stents is still reported in the range of about 25 percent to about 50 percent within six to twelve months after an angioplasty procedure. To treat this condition, additional revascularization procedures are frequently required, thereby increasing trauma and risk to the patient.
Conventional stents with surface coatings of various beneficial agents have shown promising results in reducing restenosis. U.S. Pat. No. 5,716,981, for example, discloses a stent that is surface-coated with a composition comprising a polymer carrier and paclitaxel (a well-known compound that is commonly used in the treatment of cancerous tumors). The patent offers detailed descriptions of methods for coating stent surfaces, such as spraying and dipping, as well as the desired character of the coating itself: it should “coat the stent smoothly and evenly” and “provide a uniform, predictable, prolonged release of the anti-angiogenic factor.” Surface coatings, however, may provide little actual control over the release kinetics of beneficial agents. These coatings are necessarily very thin, typically 5 to 8 microns deep. The surface area of the stent, by comparison is very large, so that the entire volume of the beneficial agent has a very short diffusion path to discharge into the surrounding tissue. The resulting cumulative drug release profile is characterized by a large initial burst, followed by a rapid approach to an asymptote, rather than the desired “uniform, prolonged release,” or linear release.
Increasing the thickness of the surface coating has the beneficial effects of improving drug release kinetics including the ability to better control drug release and to allow increased drug loading. However, the increased coating thickness results in increased overall thickness of the stent wall. This is undesirable for a number of reasons, including potential increased trauma to the vessel lumen during implantation, reduced flow cross-section of the lumen after implantation, and increased vulnerability of the coating to mechanical failure or damage during expansion and implantation. Coating thickness is one of several factors that affect the release kinetics of the beneficial agent, and limitations on thickness thereby limit the range of release rates, durations, and the like that may be achieved.
Surface coatings may also limit the delivery of multiple drugs from a stent. For example, if multiple drugs were to be released from a surface coating, the release rates, delivery periods and other release characteristics may not be independently controlled in a facile way. However, restenosis involves multiple biological processes and may be treated most effectively by a combination of drugs selected to act on these different biological processes.
A recent paper titled “Physiological Transport Forces Govern Drug Distribution for Stent-Based Delivery” by Chao-Wei Hwang et al. has revealed an important interrelationship between the spatial and temporal drug distribution properties of drug eluting stents, and cellular drug transport mechanisms. In pursuit of enhanced mechanical performance and structural properties stent designs have evolved to more complex geometries with inherent inhomogeneity in the circumferential and longitudinal distribution of stent struts. Examples of this trend are the typical commercially available stents which expand to a roughly diamond or polygonal shape when deployed in a bodily lumen. Both have been used to deliver a beneficial agent in the form of a surface coating. Studies have shown that lumen tissue portions immediately adjacent to the struts acquire much higher concentrations of drug than more remote tissue portions, such as those located in the middle of the “diamond” shaped strut cells. Significantly, this concentration gradient of drug within the lumen wall remains higher over time for hydrophobic beneficial agents, such as paclitaxel or rapamycin, which have proven to be the most effective anti-restinotic to date. Because local drug concentrations and gradients are inextricably linked to biological effects, the initial spatial distribution of the beneficial agent sources (the stent struts) is key to efficacy.
In addition to sub-optimal spatial distribution of beneficial agents, there are further potential disadvantages with surface coated stents. Certain fixed matrix polymer carriers frequently used in the device coatings typically retain a significant percent of the beneficial agent in the coating indefinitely. Since these beneficial agents are frequently highly cytotoxic in the case of paclitaxel, sub-acute and chronic problems such as chronic inflammation, late thrombosis, and late or incomplete healing of the vessel wall may occur. Additionally, the carrier polymers themselves are often inflammatory to the tissue of the vessel wall. On the other hand, the use of bio-degradable polymer carriers on stent surfaces may result in “mal-apposition” or voids between the stent and tissue of the vessel wall after the polymer carrier has degraded. The voids permit differential motion between the stent and adjacent tissue. Resulting problems include micro-abrasion and inflammation, stent drift, and failure to re-endothelialize the vessel wall.
Early human clinical trials suggest that there may be further potential problems with first generation drug delivery devices. Follow-up examination of clinical trial patients at 6 to 18 months after drug coated stent implantation indicates that mal-apposition of stent struts to arterial walls and edge effect restenosis may occur in significant numbers of patients. Edge effect restenosis occurs just beyond the proximal and distal edges of the stent and progresses around the stent edges and into the interior (luminal) space, frequently requiring repeat revascularization of the patient.
Another potential problem is that expansion of the stent may stress an overlying polymeric coating causing the coating to peel, crack, or rupture which may effect drug release kinetics or have other untoward effects. These effects have been observed in first generation drug coated stents when these stents are expanded to larger diameters, preventing their use thus far in larger diameter arteries. Further, expansion of such a coated stent in an atherosclerotic blood vessel will place circumferential shear forces on the polymeric coating, which may cause the coating to separate from the underlying stent surface. Such separation may again have untoward effects including embolization of coating fragments causing vascular obstruction.